Method for implementing a communication transceiver impairment emulator

ABSTRACT

A method for emulating signal impairments to enable dynamic evaluation of transmit and receive modem performance through the use of computer-generated models enabling both an evaluation of system performance as well as a comparison of results obtained from system designs respectively exposed to both impaired and unimpaired conditions to enable direct comparison as well as comparison with standardized measurement values to facilitate system design activities prior to any hardware implementation.

CROSS REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims priority from provisional application No.60/344,311 filed Dec. 20, 2001 which is incorporated by reference as iffully set forth.

BACKGROUND

[0002] The present invention relates to communications, communicationnetworks and especially wireless type networks. More particularly thepresent invention relates to a method for evaluating network design andcharacteristics through the introduction of impairments to the networkand enable more efficient and cost effective testing and evaluation.

DESCRIPTION OF THE RELATED ART

[0003] A communication system typically transmits an information signalfrom a source to a destination over a medium, which may be guided orunguided such as copper, optical fiber or air, the medium being commonlyreferred to as the communication channel. The information signal isaltered, i.e., modulated, to match the characteristics of the channel.The communication is demodulated at the receiving end to recover theinformation-bearing signal. The communication system typicallycompromises a transmit modem, an up converter or transmitter,communication medium, down converter or receiver and a received modem.The input data is modulated and up converted on to a predefined carrierfrequency and outputted to the communication medium. Inverse operationsare performed at the receiver.

[0004] Modulation techniques presently in use include frequencymodulation (FM), frequency shift keying (FSK), phase shift keying (PSK),binary phase shift keying (BPSK) and differential phase shift keying(DPSK). The most commonly used high speed methods for data modulationare quadrature amplitude modulation (QAM) and quadrature phase shiftkeying (QPSK). These techniques modify the amplitude and phase of apredefined carrier frequency according to an input signal in order totransmit multiple bits per baud to make more efficient use of availablebandwidth.

[0005] Modulation, such as quadrature modulation is typically performedin a modem, providing a baseband output whereupon a predefined carrierfrequency is modulated with the baseband output and is amplified andtransmitted in the communication medium. Up conversion is utilized whenchannel frequencies are above the base band frequencies. Phasemodulation techniques must be capable of overcoming phasesynchronization problems. For example, the I and Q channels employed inquadrature modulation must have the same gain, since mismatched signalgains or magnitudes create processing errors. Phase differences betweenthe carrier waveform signals cause spillover between individual channelsresulting in degraded performance. These impairments are a commonoccurrence and are due in part to the electronic mixers, filters, a/dconverters and so forth employed in up and down converters. Each of thecomponents contribute their own variations in specified value due, forexample, to temperature, manufacturing tolerances and other factorsaffecting signal integrity.

[0006] Impairments with linear behavior are encountered and arecharacterized by changes in output gain or phase which are independentof the magnitude of the input signal:

[0007] a) Amplitude imbalance

[0008] b) Phase imbalance

[0009] c) Phase jitter

[0010] d) Carrier frequency offset (receiver only)

[0011] e) Carrier leakage (transmitter only)

[0012] f) Gain ripple

[0013] g) Phase ripple

[0014] Non-linear impairments are also encountered and are characterizedby changes in output gain or phase, which vary in dependence uponmagnitude of the input signal. Two major signal impairments include:

[0015] a) amplitude-to-amplitude (AM-AM) distortion caused bynonlinearities in the overall amplifier gain transfer function and

[0016] b) amplitude-to-phase distortion (AM-PM conversion) distortioncaused by amplitude dependent phase shifts (transmitter only).

[0017] In addition to the impairments encountered during up and downconversion, the communication media, whether guided or unguided is alsoinfluenced by obstacles, attenuation and wave reflections whichperturbations affect signal level by many dB and are continuallychanging in a mobile communication environment. The propagationcharacteristics vary widely depending upon whether a communication linkis fixed or mobile, the condition of the propagation path and thecomposition of the medium itself.

[0018] When designing and prototyping new communication systems basebandmodulations/demodulation components are routinely and thoroughly testedas well as up/down conversions to and from the transmission channeloperating frequencies. Prior art testing techniques typically comprisesignal generators, E_(b)/N₀ (i.e., ratio of carrier of bit energy tonoise energy) generators and meters, channel emulators and so forth.However this method does not include conversion components.

[0019] In addition thereto it is highly desirable to be capable ofdifferentiating between up/down conversion and transmission channelimpairments from algorithmic or other systemic deficiencies and furtherto be capable of evaluating designs and modifying such designs, if andwhen necessary, prior to actual hardware implementation includingprototype implementation thereby providing a method which providessignificant time and cost efficiencies.

SUMMARY OF THE INVENTION

[0020] The present invention provides a method for emulating signalimpairments to enable dynamic evaluation of transmit and receive modemperformance through the use of computer-generated models enabling bothan evaluation of system performance as well as a comparison of resultsobtained from system designs respectively exposed to both impaired andunimpaired conditions to enable direct comparison prior to any hardwareimplementation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The objects and advantages of the present invention will becomeunderstood from the following detailed description and drawings whereinlike elements are designated by like numerals and, wherein:

[0022]FIG. 1 is a diagram showing a simplified transmitter useful inexplaining the methodology of the present invention.

[0023]FIG. 2 shows a simplified uplink receiver useful in explaining themethodology of the present invention.

[0024]FIG. 3 shows a simplified downlink receiver useful in explainingthe methodology of the present invention.

[0025]FIG. 4 is a plot showing the time domain representation of phaseripple derivation.

[0026]FIG. 5 is a diagram showing a phase ripple model.

[0027]FIG. 6 is a plot showing the time domain representation of gainripple derivation.

[0028]FIG. 7 is a block diagram showing a gain ripple model.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0029] The models developed were coded them in C and imported into testbenches built in Cadence's Signal Processing WorkSystem simulationenvironment. The models developed allow introduction of number ofdifferent radio impairments into a simulation environment that modelsthe baseband physical layer. While the designers used the Cadence tooland coded the model in C code for this implementation, the samemethodology would be applicable to different modeling environments andcoding languages. Also the designers studied the effect on the 3G TDDsignal but again the methodology and models could be used in othermodulation schemes.

[0030] As implemented, the radio impairment block (15 shown in FIGS. 1,33 and 36 shown in FIGS. 2 and 64 shown in FIG. 3) includes a parameterscreen, such as a touch screen, not shown for purposes of simplicity,which allows the operator to select those impairments to include and toset the values for each impairment to be included.

[0031]FIG. 1 shows a test model in which quadrature phase shift keyed(QPSK) data is generated at 11 and undergoes finite impulse responsefiltering at 12 and 14. An impairment is introduced at 15. Theimpairments which are introduced are set forth in detail below. The peakto average ratios (PARs) are measured and compared at 17.

[0032] Receive FIR filtering on the transmitted signals is performed at13 and 16 and the filtered signals are measured and compared for errorvector magnitude (EVM), peak code domain error (PCDE), etc. at 18. Thistest module evaluates a non-ideal transmitter in the absence andpresence of various impairments. The FIR filtering may be modified toless than ideal parameters to determine their effects on the transmittedsignal with and/or without impairments.

[0033]FIG. 2 shows an uplink receiver test module 30 in which user QPSKdata is combined at 31 with its own cell interference and multipathfading; and filtered by transmit FIRs at 32 and 35. Other cellinterference such as TDD interference from one or more neighboring cellswith different scrambling codes is introduced at 40 and impairments areintroduced at 33 and 36. Although the same impairments are provided, thesettings of the impairments provided at 33 and 36 could be different forthis test module with receiver diversity. The resultant signals arefiltered by receiver FIR filters 34 and 37 and then undergo functionsperformed by a receiver, such as demodulation, amplification, etc.

[0034] The signals are then measured at 39, testing for block error rate(BLER) raw bit error rate (BER), etc. Non-ideal shaping filters of bothtransmit and receive type may also be modeled to determine how theyaffect design.

[0035] The module 60 in FIG. 3 examines the result of downlink receiverimpairment wherein the user QPSK data connection, interferenceconnection and multipath fading are combined at 61.

[0036] Filtering is performed at 63 by simulation of a transmit FIRfilter. Other cell impairments are introduced at 62. The filtered, QPSKdata and other cell interference are combined together with impairmentsintroduced at 64. The “transmitted” signal undergoes filtering byreceiver FIR filters simulated at 65. The functions normally performedon the received signals by a receiver are simulated at 66. The outputsfrom 66 are measured at 67 and includes BLER, raw BER,signal-to-interference ratio (SIR) estimate, etc.

[0037] A working definition and description of each impairment is setforth below.

[0038] Linear impairments include amplitude imbalance, phase imbalance,phase jitter, carrier leakage/suppression, carrier offset, and dcoffset, each of which is described herein below.

[0039] Amplitude imbalance is a condition in the receiver/transmitterwherein the gain of the I and Q channels are not equal. The mathematicalmodel for amplitude imbalance is as follows:

I′=I·{square root}{square root over (2)}·cos (π/4+X)

Q′=Q·{square root}{square root over (2)}·sin (π/4+X)

[0040] Where I′=the impaired value of I,

[0041] Q′=the impaired value of Q, $\begin{matrix}{X = {{{imbalance}\quad {control}} = {{\tan^{- 1}\left( 10^{{- \delta}/20} \right)} - {\pi/4}}}} \\{\delta = {{{imbalance}\quad {in}\quad {dB}} = {{10\quad {\log \left( \frac{I_{err}}{Q_{err}} \right)}^{2}} = {10\quad {\log \left( \frac{\sqrt{2}{\cos \left( {{\pi/4} + X} \right)}}{\sqrt{2}{\sin \left( {{\pi/4} + X} \right)}} \right)}^{2}}}}}\end{matrix}$

[0042] Software limits are defined for amplitude imbalance modelparameters. The range is preferably limited to δ=+/−3 dB.

[0043] Phase imbalance is a condition in the receiver/transmitter wherethe insertion phase between I and Q channels is offset from the expected90 degrees. The mathematical model is:

I′=I·cos (φ)+Q·sin (φ)

Q′=Q·cos (φ)+I·sin (φ)

[0044] Where I′=the impaired value of I,

[0045] Q′=the impaired value of Q,

[0046] φ=phase error in degrees.

[0047] Software limits are defined for phase imbalance model parameters.The range is preferably limited to φ=+/−15 degrees.

[0048] Phase Jitter is a condition where the noise generated inside anamplifying device is manifested as a small amount of Gaussian noisemodulating the phase between I and Q channels. The mathematical model is

I′=I·cos (φ)+Q·sin (φ)

Q′=Q·cos (φ)−I·sin (φ)

[0049] Where I′=the impaired value of I,

[0050] Q′=the impaired value of Q,

[0051] φ=φ₀·random Gaussian noise

[0052] =phase error in degrees modulated by Gaussian noise

[0053] ranging between −1 and 1. The phase noise data is filtered to liein the band of 2-10 kHz.

[0054] φ₀=phase error in degrees.

[0055] Software limits are defined for phase jitter model parameters.The range is limited to φ₀=0 to 5 degrees.

[0056] Carrier leak/suppression is a condition created due to slight DCoffsets inside the quadrature modulators and has the effect of creatingadditional intermodulation distortion or reducing carrier suppression.The mathematical model is

I′=I·{square root}{square root over ((1−k))}+I _(cl)

Q′=Q·{square root}{square root over ((1−k))}+Q _(cl)

[0057] Where I′=the impaired value of I,

[0058] Q′=the impaired value of Q,

[0059] I_(cl)=k·cos (φ)

[0060] Q_(cl)=k·sin (φ)

[0061] φ=carrier leakage phase angle in degrees.

[0062] ε=20 log(k)

[0063] →k=10^(ε/20)

[0064] ε=carrier leakage in dB below full scale.

[0065] Software limits are defined for carrier leakage/suppression modelparameters. Range for magnitude is limited to ε>12 dB, applied as aloss. Range for phase angle is limited to 0<φ<360 degrees.

[0066] Carrier offset is a condition where the carrier (i.e., localoscillator) is not exactly equal to the programmed frequency. Themathematical model is

I′=I·cos (φ)+Q·sin (φ)

Q′=Q·cos (φ)−I·sin (φ)

[0067] Where I′=the impaired value of I,

[0068] Q′=the impaired value of Q,

[0069] φ=cumulative phase error in degrees across datablock=φerrCarrOffset.

[0070] errCarrOffset=2π·carrOffsetHz/sampleRate.

[0071] carrOffsetHz=carrier offset in Hertz.

[0072] sampleFreq=chipFreq* txFIRoutSampleRate=3.84 MHz*5.

[0073] chipFreq=3.84 MHz for TDD.

[0074] txFIRoutSampleRate=typically 5 for TDD for impairment appliedbetween tx & rx FIRs.

[0075] Software limits are defined for carrier offset model parameters.The range is limited to carrOffsetHz=+/−10 KHz.

[0076] DC offset is a condition in the receiver created due to slight DCoffsets and has the effect of creating bias on the inphase andquadrature components of the signal. The mathematical model is

I′=I+I _(dcoff)

Q′=Q+Q _(dcoff)

[0077] Where I′=the impaired value of I,

[0078] Q′=the impaired value of Q,

[0079] I_(dcoff)=dcOffI/100.0

[0080] Q_(dcoff)=dcOffQ/100.0

[0081] dcOffI=DC offset for I component as percentage of full scale(assumed to be 1.0).

[0082] dcOffQ=DC offset for Q component as percentage of full scale(assumed to be 1.0)

[0083] Software limits are defined for independent control of for I andQ DC offset model parameters. The range for each DC offset is limited to30.0 percent. Common mode DC offset can be simulated by setting dc OffI=dc Off Q.

[0084] Non-linear impairments include AM-to-AM distortion and AM-to-PMdistortion.

[0085] AM-to-AM distortion is an amplifier non-linearity condition wherethe output amplitude is not exactly proportional to the input amplitude,which condition typically occurs near or at the maximum output level ofthe amplifier. The mathematical model is

I′=I·(1−k·(I ² +Q ²))

Q′=Q·(1−k·(I ² +Q ²))

[0086] Where I′=the impaired value of I,

[0087] Q′=the impaired value of Q,

[0088] k=coefficient of non-linearity for the am-to-am distortion.

[0089] The AM-AM distortion non-linearity coefficient, k, is related tointermodulation in dB by the following model:

[0090] Substituting I=Acos(ω₁t) and Q=Acos(ω₂t) into the above equationfor I′ and ignoring higher order products arrive at:

I′=(1-5/4k)·cos (ω₁ t)−k/2·cos (2ω₂−ω₁)

[0091] This can be thought of as putting one tone on I and another toneon Q, and getting out the fundamental tone and its third order product.Now the intermodulation is: IM=P3rd/P1st=(k/2)²/(1-5/4k)², butconsidering that k<<1 and changing to dB get:

IM=20 log (k/2)

[0092] Software limits are defined for AM-to-AM distortion modelparameters. The range for intermodulation product is limited to therange between 50 db to 20 db below signal level.

[0093] AM-to-PM distortion is an amplifier non-linearity condition wherea change to the input level causes a corresponding change in theinsertion phase. This condition typically occurs near or at a maximumoutput level of the amplifier. The mathematical model is

I′=I·cos (φ)−Q·sin (φ)

Q′=Q·cos (φ)+I·sin (φ)

[0094] Where I′ the impaired value of I,

[0095] Q′=the impaired value of Q,

[0096] φ=k·(I²+Q²)²

[0097] k=coefficient of non-linearity for the am-to-pm distortion.

[0098] The non-linearity coefficient, k, is related to degrees by thefollowing model: For AM-PM distortion, apply the same tone to bothchannels. This can be thought of as applying two equal magnitude vectorson I and Q, in which case the output should be a vector at angle 45degrees. AM-PM causes the vector to rotate from the ideal 45 degrees.Substituting I=Q=Acos(ωt) into above equations for I′ and Q′ nd assumeonly small angles using the following small angle approximations:

sin (φ)=φ, cos (φ)=1−(φ²)/2

[0099] After substitution and cleanup arrive at:

I′=(−3/2·k ²+7/2·k+1)·cos (ωt) and

Q′=(−3/2·k ²−7/2·k+1)·cos (ωt)

[0100] The angle of the vector is arctan (I′/Q′). Considering that fork=0, the angle is 45 degrees and that the error is the angle of thevector −45 degrees, the following equation can be used to represent theAM-PM distortion error in degrees:

Error (degrees)=arctan [(3·k ²−7·k−2)/(3·k ²+7·k−2)]−45

[0101] Software limits are defined for AM-to-PM distortion modelparameters. The range for error is limited to 0 to 10.0 degrees.

[0102] Filter response impairment modeling includes phase ripple (groupdelay variation), gain ripple and non-ideal shaping filters.

[0103] Phase ripple (Group Delay Variation) is a condition where thegroup delay varies across the signal bandwith. The major contributors tophase ripple are system filters.

[0104] The impairment is modeled as the product of phase impairment andan equalizer. FIG. 4 shows the time domain representation of the phaseripple derivation. Undesirable error terms have been dropped from theresult. FIG. 5 shows a graphical representation of the impairmentimplemented by a plurality of delay lines arranged in a column D, aplurality of multipliers arranged in a column K, a plurality of summingcircuits arranged in a column S and a normalization circuit N,

[0105] Where:

[0106] Delay is the delay factor for phase ripple as derived below;

[0107] f_(c)=chip frequency,

[0108] f_(r)=frequency of phase ripple,

[0109] n=delay in complex samples,

[0110] τ=period of phase ripple=n/f_(s),

[0111] m=fir sampling rate,

[0112] f_(s)=sampling frequency=m·f_(c),

[0113] fi=bandwidth of interest=f_(s)/2,

[0114] f_(r)=1/τ=f_(s)/n=m·f_(c)/n,

[0115] n=f_(s)/f_(r)=m·f_(c)/f_(r),

[0116] Delay=2·n

[0117] k is the group delay coefficient as derived below;

[0118] T_(GDV)=peak to peak group delay, typically in units ofnanoseconds;

[0119] T_(GDV)=4·τ·k=4·n·k/f_(s),

[0120] k=T_(GDV)·f_(r)/4

[0121] The following empirically derived normalization term is appliedto resulting signal;

norm=1−k ² +k ³/8+k ⁴/2+k ⁵/2−0.00001

[0122] Software limits are defined for phase ripple model parameters.The range for ripple frequency is limited to 120 to 960 KHz. The rangefor peak-to-peak group delay is limited to the range from 1 to 600 nanoseconds.

[0123] Gain ripple is a condition where the gain varies across thesignal bandwith. The major contributors to gain ripple are systemfilters.

[0124]FIG. 6 is a time domain representation of gain ripple derivation.The impairment is modeled as shown in FIG. 7,

[0125] Where:

[0126] Delay is the delay factor for gain ripple as derived below;

[0127] f_(c)=chip frequency,

[0128] f_(r)=frequency of gain ripple,

[0129] n=delay in complex samples,

[0130] τ=period of gain ripple=n/f_(s),

[0131] m=fir sampling rate,

[0132] f_(s)=sampling frequency=m·f_(c),

[0133] fi=bandwidth of interest=f_(s)/2,

[0134] f_(r)=1/τ=f_(s)/n=m·f_(c)/n,

[0135] n=f_(s)/f_(r)=m·f_(c)/f_(r),

[0136] Delay=2·n

[0137] k is the gain ripple coefficient as derived below;

[0138] R=peak to peak ripple amplitude,

[0139] R=2019 log [1+2·k)/(1−2·k)],k = 1/2 ⋅ [(10^(R/20) − 1)/(10^(R/20) + 1)]

[0140] Software limits are defined for gain ripple model parameters. Therange for ripple frequency is limited to 120 to 960 KHz. The range forpeak-to-peak ripple amplitude is limited to the range form 0.2 to 2.0dB.

[0141] It should be noted hereinabove the model includes frequency as aninput parameter but review of the equation for k set forth above showsno frequency dependence for the gain ripple as modeled.

[0142] Non-ideal pulse shaping filters can contribute significantly toadjacent channel leakage power ratio (ACLR), error vector magnitude(EVM), peak code domain error (PCDE). By defining two signal paths inthe test environment a set of non-ideal FIR filter taps can be comparedto an ideal set of FIR filter taps to study the EVM and PCDE impact ofnon-ideal pulse shaping filters.

[0143] The tests described above may be conducted to simulate wired orwireless communications by introducing impairments respectivelyencountered in wired and wireless communications, wherein wiredcommunications include fiber optic, copper or other conductive cables,coaxial cable and the like.

What is claimed is:
 1. A method for evaluating a transmitter designed for use in a digital communication system, comprising: a) modeling a transmitter output for employment in first and second testing channels; b) introducing at least one impairment into at least one of said channels; and c) comparing the channels to determine an effect of the impairment on a transmitter output to aid in transmitter design.
 2. The method of claim 1 wherein said impairment is a linear impairment.
 3. The method of claim 1 wherein said impairment is a non-linear impairment.
 4. The method of claim 1 wherein at least one linear impairment and at least one non-linear impairment is introduced.
 5. A method for evaluating a receiver designed for use in a digital communication system, comprising: a) modeling a receiver output for employment in first and second testing channels; b) introducing at least one impairment to one of said channels; and c) comparing the channels to determine an effect of the impairment on a receiver output to aid in receiver design.
 6. The method of claim 5 wherein said impairment is a linear impairment.
 7. The method of claim 5 wherein said impairment is a non-linear impairment.
 8. The method of claim 1 comprising wherein step (b) includes modeling a baseband signal in I/Q complex representation.
 9. The method of claim 1 wherein step (b) includes modeling of amplitude imbalance RF impairment at baseband.
 10. The method of claim 1 wherein step (b) includes modeling of phase imbalance RF impairment at baseband.
 11. The method of claim 1 wherein step (b) includes modeling of phase jitter RF impairment at baseband.
 12. The method of claim 1 wherein step (b) includes modeling of carrier leakage RF impairment at baseband.
 13. The method of claim 5 wherein step (b) includes modeling of carrier offset RF impairment at baseband.
 14. The method of claim 1 wherein step (b) includes modeling of DC offset RF impairment at baseband.
 15. The method of claim 1 wherein step (b) includes modeling of phase ripple RF impairments at baseband.
 16. The method of claim 1 wherein step (b) includes modeling of gain ripple RF impairment at baseband.
 17. The method of claim 1 wherein step (b) includes modeling of amplitude modulation-amplitude modulation distortion RF impairment at baseband.
 18. The method of claim 1 wherein step (b) comprises modeling of amplitude modulation-phase modulation distortion RF impairment at baseband.
 19. The method of claim 1 further comprising evaluating non-ideal finite impulse response (FIR) filtering modeling.
 20. The method of claim 1 further including modeling at least one impairment encountered in a wireless environment.
 21. The method of claim 1 further including modeling at least one impairment encountered in a wired environment.
 22. The method of claim 14 wherein a phase ripple is implemented through the employment of delay functions, multiplier functions, summing functions and a normalization function.
 23. The method of claim 15 wherein a gain ripple impairment is implemented through the employment of delay functions, multiplier functions and summing functions.
 24. A method for evaluating a transmitter designed for use in a digital communication system, comprising: modeling a transmitter output for employment in a testing channel; introducing at least one impairment to said channel; and determining an effect of the impairment on a transmitter output to aid in transmitter design.
 25. A method for evaluating a receiver designed for use in a digital communication system, comprising: modeling a receiver output for employment in a testing channel; introducing at least one impairment into said channel; and determining an effect of the impairment on a receiver output to aid in receiver design.
 26. The method of claim 24 wherein said modeling is coded in C language.
 27. The method of claim 25 wherein said modeling is coded in C language.
 28. The method of claim 26 wherein models are imported into test benches which provide a desired simulation.
 29. The method of claim 24 wherein the modeling is at baseband.
 30. The method of claim 5 wherein the modeling is at baseband.
 31. The method of claim 5 wherein step (b) includes modeling of amplitude imbalance RF impairment at baseband.
 32. The method of claim 5 wherein step (b) includes modeling of phase imbalance RF impairment at baseband.
 33. The method of claim 5 wherein step (b) includes modeling of phase jitter RF impairment at baseband.
 34. The method of claim 5 wherein step (b) includes modeling of phase ripple RF impairments at baseband.
 35. The method of claim 5 wherein step (b) includes modeling of gain ripple RF impairment at baseband. 